The present invention relates in general to a recording/reproduction technique for a digital audio signal such as PCM signal. In particular, the invention is concerned with a method of recording/reproducing an encoded audio signal singly or together with a video signal onto/from a magnetic tape by means of a rotary magnetic head type scanner. The invention also concerns an apparatus for carrying out the method.
In an effort to enhance the quality of audio signal which accompanies a video signal, there have been developed and adopted in practical applications recording/reproducing techniques for the PCM audio signals.
For example, an 8 mm video recorder adopts an audio PCM system. A sampling frequency of the audio PCM signal is 31.5 KHz which is two times as high as a repetition frequency of a horizontal synchronizing signal, and differs from an internationally standardized sampling frequency (32 KHz, 44.1 KHz or 48 KHz). By way of example, a sampling frequency of an audio PCM signal in a satellite broadcast is 32 KHz or 48 KHz.
On the other hand, in a MUSE system which is one of transmission systems for a high definition television system, the sampling frequency of the audio PCM signal is 32 KHz or 48 KHz. Thus, when the data sampled at the above sampling frequency are to be recorded field by field (i.e. on the field basis), the number of data per field includes a fraction, to inconvenience. In order to solve this problem, a packet transmission system having a leap field to absorb the excess has been adopted, as disclosed in NHK GIKEN MONTHLY REPORT 27-7, page 282.
In a video disk system, the PCM audio signal sampled at the sampling frequency of 44.1 KHz is recorded in the same format as that of a compact disk.
However, when the PCM signal is to be recorded by an apparatus such as a video tape recorder which records or transmits the signal discontinuously in time, a following problems arise. First, when a field frequency of a video signal is not an integer multiple of the sampling frequency of the audio signal, the problem described above is encountered in the coding. In the MUSE system proposed for solving the problem, there must be a synchronous relationship between the field frequency f.sub.V of the video signal (or a rotation frequency f.sub.D of a head scanner which rotates synchronously with the field frequency) and the sampling frequency f.sub.S of the video signal. This imposes a limitation to a system application range.
Further, when the signal recording format developed for the MUSE system is adopted in the video tape recorder system, there will arise a problem that due to insufficient capability of correcting data error brought about by injuries of tape and/or dusts deposited on the tape, adequate reliability can not be assured in the reproduction of audio signal. To cope with this problem, correcting capability of the correction code may be correspondingly reinforced while the burst error may be corrected through interleave with increased distance. In that case, however, the time required for the processing becomes as long as about 30 msec per video field due to the orthogonal interleave, involving difference in time between the video signal and the PCM audio signal, which in turn results in that viewers might feel discomfort, to another problem.
As an apparatus known heretofore for PCM recording/reproducing only the audio signal by a rotary head type VTR, there can be mentioned a Consumer-Use PCM Encoder-Decoder according to the Electronic Industries Association of Japan Technical Standard CPZ-105 (September, 1983). A typical one of the recording/reproduction apparatus based on this technical standards is shown in FIGS. 1 and 14 of an article entitled "Digital Audio/Video Combination Recorder Using Custom Made LSI's, IC's" presented at the 69th Convention 1981 May 12-15 Los Angeles AES 1791 (B-6). According to the teachings disclosed in this literature, the field frequency f.sub.V and the sampling frequency f.sub.S, for example, in the NTSC system are derived from a same master clock signal in such a relationship that f.sub.S =75 f.sub.V. Accordingly, the number of samples per field is constant at 735 samples.
An arrangement of the sampled audio signal recording/reproducing system is shown in FIG. 1 of the above-mentioned literature. From this figure, it is seen that the address of an RAM serving as an interleave memory is controlled by an address control circuit.
The proposal disclosed in the above literature is based on the assumption that the field frequency f.sub.V bears a predetermined relationship to the sampling frequency f.sub.S, and no consideration is paid for the case where no correlation exists between the field frequency f.sub.V and the sampling frequency f.sub.S.
In the prior art techniques described above, the sampling frequency of the video signal is not the internationally standardized sampling frequency, the number of quantized bits is small, and the sampling frequency of the audio signal and the field frequency are required to have a synchronized relationship. Thus, when a video signal from a camera and a digital signal from a compact disk (CD) system are to be recorded in combination, a great difficulty is encountered in recording them simultaneously because the sampling frequencies are different and there is no synchronous relationship between the sampling frequency and the field frequency.
Further, the audio PCM signal takes a lot of time for error correcting processing as well as for the interleave processing for dispersing data so that burst error possibly existing on the tape does not provide successive errors. Consequently, in case the video signal processed analogically and the PCM audio signal are recorded simultaneously, there arises a problem that the audio signal undergoes time lag due to the time taken for the processing for deinterleave and error detection/correction. Under the circumstance, the audio signal processing system having the processing time reduced to be as short as possible is required. Besides, for solving the problems mentioned above, the circuit scale has to be minimized with a much simplified circuit configuration.